The contest is open to people living in the U.S., as well as Australia, Austria, Belgium, Canada (excluding the Province of Quebec), Denmark, Finland, France, Germany, Greece, Ireland, Italy, Japan, Luxembourg, Netherlands, New Zealand, Norway, Portugal, Spain, Sweden, Switzerland, or the United Kingdom.

Customers can enter up to 25 times per day, either by downloading an app or by visiting a separate page that negates the purchase requirement. (Edit 12:26 PM PDT: As reader "suefrost" noted, however, this isn't correct; users who use the page without the purchase requirement receive a message "You have entered to [sic] many times today" if they try to sign up more than once.)

Although the iPhone and iPod touch have been an unqualified success, a billion downloaded apps is still a spectacular milestone, as the service has been available for less than a yearPandora Radio said that over 2 million users had downloaded its free app; the service ranks third on a list of free apps that Apple has posted; Facebook and Google Earth are the top two.

And how close is the contest to being over? At press time, the counter was at about 929,080,000, or about 900 apps downloaded per second. At that rate, the contest will finish up in about 1,300 minutes, or just before 9 AM PDT on Saturday morning.

Edit 4/13: OK, either my math was way off, or apps sales significantly slowed down over the weekend. In any event, it's about 7:30 now, and the apps counter is at about 946,212,000.